Stump cutting machines are well known for removing undesirable tree stumps. Such machines include a rotating cutter wheel driven by a gas or diesel engine. The cutter wheel, while rotating, is advanced toward the stump and moved laterally across the face of the stump. The cutter wheel is mounted at one end of a boom which is, in turn, pivotally mounted on a support frame. Hydraulic boom swing cylinders are used to pivot the boom about the pivot point to move the cutter wheel back and forth across the face of the stump to cut it away. The hydraulic boom swing cylinders are controlled by control lever handles. The handles also can provide an additional function, namely determining operator presence. For this latter function, the cutter wheel is stopped if the operator does not maintain his or her hand on the handle(s) during operation.
Using trigger switches and other spring biased devices employed on the operator handles to detect operator presence creates difficulties for the operator which include, among other things, operator fatigue by requiring that the operator's hands remain in a fixed position to engage the biased device over long periods of time while manipulating the cutter wheel positioning controls numerous times using one or both hands. In order to relieve these difficulties, capacitive sensing on the handles has also been used. FIG. 1 illustrates this type of prior art system.
More specifically, the capacitive sensing devices in FIG. 1 include a ring sensor 6 mounted in the handle 1. When the operator's hand 2 touches the handle, the capacitance C is changed. A circuit includes a wave generator 5 to provide a signal to the ring sensor 6. The circuit senses the change in capacitance and interprets the change in capacitance as detecting operator presence. One drawback of this system, is that water 7 (such as from rain or snow) touching the handle may provide a change in the capacitance—even if the operator's hand is not on the handle. Because stump cutters are used in outdoor environments, the handles are exposed to the elements and so detection errors may occur.
Accordingly, there is a need in the art for a method and apparatus to more reliably determine if an operator's hand(s) is present on the control handle while the machine is in operation. The present invention overcomes the shortcomings of the prior art, and addresses these needs.